When operating high-speed, high-capacity, local area network data communications systems (eg., in the Ethernet environment) it is often necessary to monitor data transmission with an absolute minimum of intrusion or disruption of the data stream. The data stream is usually carried as one-volt-peak-to-peak push-pull pulses on a fifty-ohm coaxial medium or on a T-type twisted pair medium running in the ten-megabaud (ten million bits per second) to one-hundred-twenty-five megabaud range. The transmission can be in either simplex or half-duplex mode (respectively, transmission can be in one direction only or in two directions but not simultaneously), or transmission can be in full-duplex mode (both directions simultaneously, usually using two oppositely-directed simplex channels).
One of the more common families of existing transmission testing apparatus or transmission analyzers (TAs) has an input impedance of about one-hundred ohms. If such a TA were connected across (in parallel with) or in series with such a 50-ohm local area network data transmission channel or line, the input impedance of the TA could seriously disrupt the transmission channel's impedance match, with potentially dire consequences at a 10-125 megabaud transmission rate. At such high digital transmission speeds, frequency bandwidths are much higher than for sinusoidal signals such as television transmission. A familiarity with Fourier series analysis will aid in understanding why multimega-baud digital transmission requires so much higher bandwidth capability than sinusoidal transmission at the same frequency numbers.
In a local area network, a group of workstations is usually connected to a transmission line or to a digital switch by a circuit called a shared media hub (SMH) which sends all traffic from one workstation to all the other workstations that it serves in addition to sending that traffic to the switch, if required. To monitor simplex or half-duplex transmission between a SMH and a digital switch, the usual practice is to connect another SMH in the transmission line for the test.
Unless the SMH is connected into the line ab initio and left permanently in place, dedicated to testing that transmission line at that node, the transmission must be interrupted and restarted each time that the testing SMH is reconnected into the transmission line so that testing can be started. The transmission would again have to be interrupted to remove the SMH for use in a test somewhere else. Temporarily taking the transmission line out of service twice whenever the line is to be tested is usually unacceptable.
Permanently installing SMHs at any number of nodes of a transmission line for testing purposes only is also unacceptable when it is considered that a SMH can cost about $3,000. Also, most SMHs, as presently available, are for simplex transmission only and would not support the very common full duplex transmission without the addition of a second SMH. Reasonable cost objectives militate against installing a plurality of paired SMHs at strategic locations in a local area network for the sole purpose of facilitating occasional testing of the network at that particular node.
The use of impedance-matching coupling transformers is widely known in the digital data transmission art for interconnecting devices and media of different impedances. However, for digital data transmission, such coupling transformers are usually designed for use over a limited band of digital transmission speeds or rates. It may be theoretically possible to custom design a coupling transformer to meet a very wide frequency bandwidth needed to transmit digital signals at rates of from ten megabaud to significantly over one-hundred megabaud. However, it would be highly unlikely that such a custom-made coupling transformer would meet cost objectives (mentioned above) in order to permit widespread permanent installation at strategic locations throughout a local area network where monitoring and transmission analysis MIGHT be desired in the future.
Impedance matching can also be accomplished using discrete passive electronic devices such as individual capacitors and inductors. However, at such digital transmission speeds, design difficulties such as component and conductor placement and orientation pose significant design difficulties which would be expected to affect adversely upon production yield.
It is theoretically possible to use operational amplifiers to present a very high input impedance to a digital transmission line and a fairly low output impedance to a transmission analyzer. However, since operation is expected to take place at one-hundred-plus megabaud speeds, the maximum frequency capability of the operational amplifier must be many times higher than the baud (bits per second) of the digital data transmission signal. Such high-frequency operational amplifiers exist. However, there are two conductors comprising each simplex transmission line. There must be a separate operational amplifier for each conductor of the transmission line in order to reproduce the positive and negative signals involved. Two operational amplifiers connected to the two conductors of a digital data transmission line must have the same maximum frequency capability. Otherwise the positive and negative transitions of the amplified digital data signals will lack and thus impair the operation of the digital transmission analyzer.